1. Field of the invention
The present invention relates to an electroluminescent device (hereinafter referred as "EL device") having an emitting layer made of an emitting substance, which utilizes an electroluminescence phenomenon that is the emission of light resulting from application of an electric field or electrons to the substance. More particularly, it is concerned with an organic EL device in which the emitting layer is made of an organic luminous substance.
2. Description of the prior art
As organic EL devices, there have been known a device of two-layer structure type having two layers made of organic compounds respectively as shown in FIG. 1, in which an organic fluorescent thin film 3 (hereinafter referred as "emitting layer") and an organic positive-hole transport layer 4 are laminated with each other and are arranged between a metal cathode 1 and a transparent anode 2. There have been also known a device of three-layer structure type having three layers made of organic compounds respectively as shown in FIG. 2, in which an organic electron transport layer 5, an emitting layer 3 and an organic hole transport layer 4 are laminated in sequence and are sandwiched as a whole between a metal cathode 1 and a transparent anode 2. (Japanese Patent Application non-examined publication Tokkai hei 2-216790)
The hole transport layer 4 facilitates the infusion of the holes from the anode and blocks electrons. The electron transport layer 5 facilitates the infusion of electrons from the cathode. In these organic EL devices, a glass substrate 6 is furnished outside the transparent anode 2. The recombination of electrons injected from the metal cathode 1 and the holes injected from the transparent anode 2 to the emitting layer 3 generates excitons. The excitons emit light when they are deactivated through radiation. This light radiates toward the outside through the transparent anode 2 and the glass substrate 6. The transparent anode 2 is formed of an electric conductive material with a high work function such as indium-tin oxide (hereinafter referred as "ITO").
The cathode 1 is formed of a metal with a lower work function such as aluminum (Al), magnesium (Mg), indium (In), silver (Ag) or alloys of the individual metals thereof Al-Mg, Ag-Mg, for example. Generally, the cathode 1 has the following desired functions. First, it efficiently injects electrons to the emitting layer, and second it is used as bus lines for electric current in an organic EL device in the form of matrix structure. It is necessary for the required first and second functions to select restrictively a cathode material having both a low work function and a low resistance so as to be formed as a thin film.
However, most conventional cathode metal materials have insufficient injecting efficiency of electrons. The EL device using such a cathode material has a decrement of luminance during running of the emitting process.